1. Field of the Invention
The present invention relates to a method for stabilizing a rhodium compound and more particularly to a method for stabilizing a rhodium compound contained in a catalyst solution contaminated with tar formed as a by-product in a carbonylation reaction in which methyl acetate or dimethyl ether is chemically bonded with carbon monoxide in the presence of a catalyst system comprising a rhodium compound and an alkali metal iodide to produce acetic anhydride.
2. Description of the Related Art
A reaction of carbon monoxide, alone or together with hydrogen, with methyl acetate or dimethyl ether in the presence of a rhodium catalyst to produce acetic anhydride is very useful from the viewpoint of industry. It is known that the addition of various co-catalysts is useful for this reaction . It is also well known that the most serious problem of this reaction resides in a lowering in reactivity due to the accumulation of tar as a by-product. For this reason, in order to conduct this reaction on an industrial scale, it is necessary to remove the tar from the system.
In the above-described reaction, since use is generally made of a homogeneous catalyst, the tar and rhodium catalyst are contained in the same solution. Therefore, the development of an efficient tar/rhodium separation process capable of minimizing the loss of the rhodium compound, which is a very expensive catalyst, is indispensable to the practice of the above-described reaction on an industrial scale.
Typical examples of the tar/rhodium separation processes of the type described above include (1) extraction with an aqueous methyl iodide/hydroiodic acid solution, (2) precipitation/insolubilization of a rhodium complex by heat treatment and (3) extraction with an alkane or a cycloalkane (see U.S. Pat. No. 4,476,238; published on Oct. 9, 1984; Halcon-SD-Group Inc.). Among these processes, the process (1) has a drawback in that, since the reaction system is contaminated with hydroiodic acid, a compound containing iodine should be removed from the reaction system for the purpose of keeping the balance between the rhodium concentration and the iodine concentration in the reaction system. The process (2) has a drawback in that this process is very complicated due to the occurrence of sedimentation of the rhodium compound as a solid. The process (3) has none of the above-described problems and enables the tar/rhodium separation to be conducted in a very simple manner (see the above described U.S. Pat. No. 4,476,238).
However, as described also in the U.S. Pat. No. 4,476,238, in the process (3), accumulation of various compounds in the extractant should be prevented. Specifically, the extractant after the extraction and the compound contained therein should be substantially separated from each other by an industrially practicable operation, such as distillation. Since, however, alkanes and cycloalkanes, for example, pentane and cyclohexane, generally form azeotropes with components having a low-boiling point and contained in large amounts in the reaction system for the above-described acetic anhydride formation, such as methyl iodide and methyl acetate, not only it is difficult to separate these compounds from the extractant by distillation but also other separation operations are also difficult to conduct on an industrial scale. For this reason, before the catalyst solution to be extracted, which is generally a concentrated catalyst solution provided by flash vaporization of a carbonylation reaction mixture (hereinafter referred to simply as "concentrated catalyst solution"), is applied to a tar extraction process using an alkane and a cycloalkane, components having a low-boiling point contained in the catalyst solution, such as methyl iodide and methyl acetate, should be separated by distillation.
However, studies conducted by the present inventors have revealed that some rhodium complexes contained in the concentrated catalyst solution are highly liable to sediment upon being heat-treated at a temperature of around the boiling point of the concentrated catalyst solution. The reason for this will now be described.
The rhodium catalyst contained in the concentrated catalyst solution has been found to comprise a mixture of the following four complexes by IR spectroscopy:
(a) [Rh.sup.+ (CO).sub.2 I.sub.2 ].sup.- (2060, 1990 cm.sup.-1), PA1 (b) [Rh.sup.3+ (CO).sub.2 I.sub.4 ].sup.- (2085 cm.sup.-1), PA1 (c) [Rh.sup.3+ (CO)I.sub.4 ].sup.- (2064 cm.sup.-1), and PA1 (d) [Rh.sup.3+ (CO)I.sub.5 ].sup.2- (2035 cm.sup.-1).
It can be easily perceived that, among these complexes, the rhodium complex (c) having an unsaturated coordination is thermally unstable. In fact, as is apparent from Reference Example 1, which will be descried later, studies conducted by the present inventors have revealed that the concentrated catalyst solution containing the rhodium complex (c) is more unstable during heat treatment than the concentrated catalyst solution not containing the rhodium complex (c). Here, it is to be noted that the rhodium complex (c) is thought to be converted into a rhodium complex (d) when it has reacted with an iodide ion.
pti [Rh.sup.3+ (CO)I.sub.4 ].sup.- +I.sup.-.fwdarw.[Rh.sup.3+ (Co)I.sub.5 ].sup.2-
Therefore, it is expected that if the concentration of the iodide ion in the concentrated catalyst solution is sufficiently high, the concentrated catalyst solution is substantially free from the rhodium complex (c), so that the concentrated catalyst solution would be stable during heat treatment. In fact, as is apparent from Reference Example 2, which will be described later, studies conducted by the present inventors have revealed that, when the molar ratio of the iodide ion to rhodium, hereinafter referred to as "I/Rh (molar ratio)", is 20 or more, the rhodium complex (c) is absent in the concentrated catalyst solution, so that the concentrated catalyst solution is stable also during heat treatment at the boiling point of the concentrated catalyst solution. Therefore, it is conceivable that, if the reaction for producing acetic anhydride is conducted in the presence of a catalyst system having such a composition that an alkali metal iodide exists in an amount of 20 mol or more per mol of a rhodium compound, i.e. the I/Rh (molar ratio) becomes 20 or more, the concentrated catalyst solution would be stable during the heat treatment. This method, however, has a serious problem.
The problem is that under the carbonylation reaction conditions, the alkali metal iodide reacts with methyl acetate within the reactor, which causes most of the alkali metal iodide to be converted into an alkali metal salt: of acetic acid, such as lithium acetate, and methyl iodide unfavorably. In this connection, the present inventors have found that, as is apparent from Reference Example 3, which will be described later, when the reaction has reached a steady state, the iodide ion concentration is apparently lower than that during the stage of initial charging. For this reason, in order to maintain the I/Rh (molar ratio) in the concentrated catalyst solution at 20 or more, it is necessary to-add the iodide considerably in excess over the necessary amount. This is disadvantageous from the viewpoints of an increase in the amount of tar formed as a by-product and an increase in the cost. Therefore, it is substantially impossible to successfully conduct the tar/rhodium separation by extraction with an alkane or a cycloalkane on an industrial scale unless the above-described problem is solved.
The addition of an alkali metal iodide or hydroiodic acid to the concentrated catalyst solution to increase the iodide ion concentration is deemed effective as one method for solving the problem. In this method, however, an alkali metal and/or iodine are added to the concentrated catalyst solution from the outside of a series of reaction systems. For this reason, in order that the concentrated catalyst solution containing the alkali metal and/or iodine added may be reprocessed and used as the catalyst for the carbonylation reaction in which methyl acetate or dimethyl ether is chemically bonded with carbon monoxide to form acetic anhydride, balances between the rhodium concentration, the alkali metal concentration and the iodine concentration in the carbonylation reaction system should be kept, so that the compounds containing these elements should be withdrawn in an amount corresponding to the amount of added elements from the concentrated catalyst solution or the catalyst solution after the catalyst reprocessing treatment. Since, however, the compounds containing alkali metals exist in the form of salts which are completely soluble in the (concentrated) catalyst solution, it is difficult to selectively remove these compounds by conventional techniques. On the other hand, the iodine-containing compound exists as methyl iodide and, therefore, can be separated by distillation. However, the separated methyl iodide is a highly toxic compound, which renders the treatment thereof very difficult.